1. Field of the Invention
The invention relates to an optical polarization splitter accomplishing the spatial splitting of two orthogonal polarization components with a wide spectral and angular acceptance. This splitter is particularly applicable in liquid-crystal displays. The use of a polarization beam splitters or PBSs makes it possible in fact to improve the luminous efficiency in transmissive liquid-crystal projector architectures or to split the beam coming from the illumination thereof, modulated and having to be projected in reflective liquid-crystal projector architectures.
2. Discussion of the Background
There are two large families of polarization splitters.
A first family uses the combination of an interference filter composed of dielectric layers and the Brewster effect. What is involved is a polarizer of the Neille type described, for example, in the document xe2x80x9cHigh Performance Thin Film Polarizer for the UV and visible spectral regionsxe2x80x9d by J. A. Dobrowolski and A. Waldorf, Applied Optics, Vol. 20, p. 111, 1981.
This technique is widely used in the polarization splitters currently available on the market. Such a splitter is generally in the form of a cube, the dielectric layers making an angle of 45xc2x0 with the normal. Their main drawback is the low angular acceptance, of about xc2x17xc2x0, this often being insufficient for display applications. The available spectral band depends on the combination of layers and may cover the entire visible spectrum.
A second family of splitters uses the total reflection of light when the latter crosses a dioptic interface, passing from a medium of index n1 to a medium of index n2 where n1 greater than n2. The angle xcex8R above which there is total reflection is obtained from Fresnel""s Law:       θ    R    =      arcsin    ⁢          xe2x80x83        ⁢                  n        2                    n        1            
For any angle xcex8 greater than xcex8R there will be total reflection.
The principle of such a splitter is shown in FIG. 1 and, for example, in French Patent Application No. 2 685 500. Two high-index prisms 1 and 2 are separated by a birefringent layer 3 composed of polymerized liquid crystal. The optical axis of the layer is coincident with the director of the elongate liquid-crystal molecule. The molecules are aligned in the plane of the liquid-crystal layer. Let ne be the extraordinary index of the liquid crystal and no the ordinary index. For nematic liquid crystals, ne greater than no. Let us assume that the indices of the two prisms are equal to ne. One of the polarizations incident on the prism/liquid-crystal layer interface at an angle   θ  =            θ      R        =          arcsin      ⁢              xe2x80x83            ⁢                        n          o                          n          e                    
will experience the index ne and will be totally transmitted, whereas the orthogonal polarization will experience no and thus be under conditions for total reflection: it is therefore totally reflected.
Let P be the polarization in the plane of incidence (the plane of the drawing in FIG. 1).
Let S be the polarization perpendicular to the plane of incidence (perpendicular to the plane of the drawing). Thus:
P experiences the index no and is totally reflected for xcex8 greater than xcex8R 
S experiences the index ne and is totally transmitted.
The numerical example given in French Patent Application No. 2 685 500 corresponds to the case of a nematic liquid crystal with:
no=1.5;
ne=1.65, the index of the 2 prisms;
xcex8R=65xc2x0 (in the prism)
to stabilize the structure, the liquid crystal is polymerized.
The angles for which polarization splitting takes place are between 62xc2x0 and 85xc2x0 (the angles from 85xc2x0 to 90xc2x0 are not counted as they are difficult to use), i.e. xc2x112xc2x0 in a medium of 1.7 index, corresponding to xc2x120xc2x0 in air. It may be seen that the angular acceptance of this component is better than that of a Brewster interference splitter. The total reflection is also independent of the wavelength xcex, provided that the indices ne and no of the layer vary little with xcex.
The invention relates to a device in which the birefringent layer is of a different nature.
The invention therefore relates to a polarization splitter comprising a splitting medium lying between two transparent elements of defined indices, characterized in that the splitting medium comprises a periodic structure of layers of materials of different indices and of a period which is small compared with the wavelength of an incident beam. This stack forms, by xe2x80x9cform birefringencexe2x80x9d, a uniaxial birefringent medium of optical axis perpendicular to the plane of the periodic structure.